Short-duration light source comprising an electron irradiated transparent insulating body



Nov. 1, 1966 v. E. CULLER 3,283,155

SHORT-DURATION LIGHT SOURCE COMPRISING AN ELECTRON IRRADIATEDTRANSPARENT INSULATING BODY Filed June 5, 1963 HIGH-ENERGY ELECTRON BEAMFIG. 1

NEGATIVE ELECTRICAL CHARGES INVENTCR. Vaughn E.Cul|er ATTORNEY UnitedStates Patent O 3,283,155 SHORT-DURATIQN LIGHT SUURCE CQMPRISKNG ANELECTRUN IRRADIATED TRANSPARENT INSULATING RUDY Vaughn E. Cutler,Pennington, N.J., assignor to Corning Glass Works, Corning, N.Y., acorporation of New York Filed June 5, 1963, Ser. No. 285,792 3 Claims.Cl. 250-84) The present invention relates to sources for producing lightpulses of very short duration and to methods for producing such sources.

An object of the invention is to provide a source of light pulses havingdurations of the order of one microsecond.

A further object is to provide such a light source which is capable ofstoring for long periods the electrical charge necessary to produce sucha light pulse.

Another object is to provide a simple and inexpensive flash source foruse in photography.

These and other objects, which will be apparent from the description,are accomplished by irradiating a transparent insulating material havingcertain electrical characteristics in order to provide a localizedexcess of electrical charge in one portion of the material andsubsequently disrupting the charge distribution to eifect an electricaldischarge which produces a light pulse.

The invention will be described with reference to the accompanyingdrawing, in which:

FIGURE 1 illustrates the process whereby a light source according to thepresent invention is produced, and

FIGURE 2 illustrates a light source in condition to be discharged toproduce a light pulse.

It has previously been known that it is possible to collect electricalcharges in insulating materials and that subsequent discharge, eitherspontaneous or intentional, may produce lightningdike flashes of light.Until the present time, however, it has not been possible to store suchcharges for times sufficient to permit the use of such materials aslight sources for commercial application, such as photography, and forthis reason such materials have been of academic interest only.Accordingly, the present invention involves both the recognition thattransparent insulating materials having localized electrical charges maybe used as light sources for photographic application and that suchmaterials, when chosen to have particular electrical characteristics,may store such charges for times sufiicient for commercial photographicuse.

It is known that charged insulators lose their charge according to theequation:

where q is the charge density at time i, g is the charge density at time0, and T is a time constant characteristic of the insulator. The sametime dependence is also known to be true for the internal electric fieldin an insulator.

The time constant can be derived as where T is in seconds, K is therelative dielectric constant and p is the resistivity of the insulatorin ohm-cm.

It is known that a material will absorb only a limited amount ofelectrical charge at a localized portion of the material beforeelectrical breakdown. In order for such material to be usable as apractical commercial light source, it is necessary for such charge, or asubstantial part thereof, to be retained for some length of time. In thecase of glasses and plastics, it has been found that for practical use,the material must retain on the order of 37% of the original depositedcharge in order for it "ice to be useful as a photographic light source.From Equation 1, it can be determined that when the time t is equal tothe time constant T, both the electrical charge and the electrical fielddecay to about 37% of their original values. Initially it was assumedthat for a charged glass or plastic material to have utility as a lightsource, a useful life of days is required. Thus, the charge and fieldmay decay to 37% of their original value in a period equal to 90 days,and will do so when the time constant T equals 90 days.

In order to calculate a preliminary value for the product K9, therequired value of the time constant T was substituted in Equation 2, anda tentative value of the product K for adequate electric chargeretention was determined as approximately 10 ohm-cm.

There is evidence that at very high electric field strengths resistivitydoes not have the same value that it has at lower field strengths. Inaddition, it is difiicult to determine the correct dielectric constantexcept with nearly ideal insulators. For these reasons, the calculatedrequired value of K was verified empirically by irradiating variousglass samples having values of K centering around 10 ohm-cm. Theexperimental results indicate that although most materials having Kpequal to or greater than 10 ohm-cm. exhibit adequate charge retention, apreferred value of at least 10 ohm-cm. is necessary in order to insure alife of 90 days in all materials.

Additional experimentation has shown that certain glasses, for examplevitreous silica, exhibit large increases in electrical conductivityduring irradiation. Such glasses, even though exhibiting the required Kpunder normal conditions, cannot be charged sufficiently for use as lightsources. It has been found that in order for a localized charge ofsuificient magnitude to be built up, it is necessary that K be greaterthan 3X10 ohm-cm. during irradiation.

Although the present invention is not to be limited thereby, thefollowing is given as a preferred embodiment of the invention:

Blocks of glass, such as 10, in the shape of right circular cylinderstwo inches in diameter and one-fourth inch thick were ground andpolished. The glass had a composition 58% SiO 15% A1 0 9% CaO, 8% MgO,6% R210 and 4% B 0 The extrapolated room temperature resistivity of theglass was 10 ohm-cm., and its dielectric constant was about 6.4.

One flat side of each block was exposed to 2 mev.

electron irradiation. The total exposure was 1.2 megarads, and wasattained at 0.1 megarad per pass through the electron beam. Afterexposure the blocks were stored for periods up to six months. Eachblock, when removed from storage and struck with a metal rod, emitted abright flash of light having a duration which was measured to be aboutone microsecond.

When a single irradiated block was discharged 15 /2 inches from a 12inch diameter disc rotating at 3650 revolutions per minute and havingthereon radial lines separated by 0.45 mm., a photograph taken at acamera setting of f/2 with a film having a speed rating of 800 ASAclearly resolved the radial lines.

Glass blocks irradiated according to the present invention exhibit overconventional high-speed flash sources the advantages of compact size andlow cost.

Light sources according to the present invention may be producedutilizing any transparent material having the described electricalcharacteristics. Localized concentrations of electrical charges may beproduced by irradiation with electrons, as in the described embodiment,by irradiation with gamma rays, which cause electrons in the material tomove away from the radiation source, or by irradiation with protons.

The light emitted from electron-irradiated glass blocks is essentiallywhite light and can be used for taking satisfactory color photographs.It is of interest that, although such irradiated glass blocks pick uppositive charges from the atmosphere and are accordingly electricallyneutral, if such blocks have the above-described electricalcharacteristics, the localized electrical fields necessary for theproduction of light pulses remain.

When blocks larger than those described above are utilized, both themagnitude and the duration of the resultant light pulse are increased,and blocks of varying shapes may be used, as dictated by considerationssuch as reflector shape when such blocks are utilized in photography.

It is to be understood that although the above-described limitations onthe value of the quantity K are necessary if light sources according tothe present invention are to be used for conventional photographicapplications, there will be some instances in which substantial storagetimes are not required, and in such instances the normal value of thequantity Kp may be less'than 10 ohm-cm. In all cases, however,regardless of the required storage time, the value of Kp duringirradiation must be at least equal to 3 10 ohm-cm.

It will be understood that the foregoing has been given merely as adescription of a preferred embodiment of the present invention.Accordingly, it is intended that the invention be limited only by thescope of the appended claims.

What is claimed is:

1. A light source comprising a transparent material having the normalproduct of its electrical resistivity and its dielectric constant equalto at least 10 ohm-cm. and

having during electron irradiation the product of its electricalresistivity and dielectric constant equal to at least 3 10 ohm-cm, saidmaterial having concentrated in a portion thereof an excess of negativeelectrical charges.

2. A light source comprising a glass body having the normal product ofits electrical resistivity and its dielectric constant equal to at least10 ohm-cm. and having under electron irradiation the product of itselectrical resistivity and its dielectric constant equal to at least 310 ohm-cm, said body having concentrated in a portion thereof an excessof negative electrical charges.

3. The method for producing a light source which comprises irradiationwith electrons one part of a transparent insulating body normally havingthe product of its electrical resistivity and its dielectric constantequal to at least 10 ohm-cm. and having during irradiation the productof its electrical resistivity and its dielectric constant equal to atleast 3 10 ohm-cm. in order to build up in said part of said body anexcess of negative electrical charge.

References Cited by the Examiner UNITED STATES PATENTS 2,990,473 6/1961Kallmann 2507l 3,093,735 6/1963 Blakewood et al 25083 3,165,631 1/1965Solow et al 25042 RALPH G. NILSON, Primary Examiner.

H. S. MILLER, A. R. BORCHELT, G. E. MATTHEWS,

Assistant Examiners.

2. A LIGHT SOURCE COMPRISING A GLASS BODY HAVING THE NORMAL PORDUCT OFITS ELECTRICAL RESISTIVITY AND ITS DIELECTRIC CONSTANT EQUAL TO AT LEAST1022 OHM-CM. AND HAVING UNDER ELECTRON IRRADIATION THE PRODUCT OF ITSELECTRICAL RESISTIVITY AND ITS DIELECTRIC CONSTANT EQUAL TO AT LEAST3X1016 OHM-CM., SAID BODY HAVING CONCENTRATED IN A PORTION THEREOF ANEXCESS OF NEGATIVE ELECTRICAL CHARGES.